PAGE No. 40

July 1960

This addition to our range of standard single-unit P.A.B.X. equipments caters for requirements not exceeding 50 extension lines and 10 exchange lines. Used in conjunction with an associated cordless switchboard it offers a varied range of facilities. It can be provided with B.P.O. 2000 or 4000 type two-motion selectors.

NEW versions of private automatic branch exchanges that provide up-to-date facilities and features of durability to improve service and minimize maintenance are essential to the working efficiency of many modern business organizations.

To augment our range of private automatic branch exchanges designed to fulfil such requirements, the Company has introduced a P.A.B.X. having capacity for 50 extension lines and 10 exchange lines. The equipment, as shown in Fig. 1 (right), includes a small, attractive cordless switchboard coupled with a marker-switching arrangement to enable the attendant to deal with incoming exchange calls and other demands promptly and efficiently.

The extension line circuits are provided with a lock-out facility to prevent switching equipment being held unnecessarily under P.G. or C.S.H. alarm conditions. By means of simple connection strip strapping any extension may be placed in one of the following categories:-

(a) An extension having direct access to the public exchange and right-of-way (intrusion) facilities
on internal calls.

(b) An extension having direct access to the public exchange but without right-of-way facilities.

(c) An extension barred from direct access to the public exchange but permitted to receive incoming or transferred public exchange calls. Outgoing calls are dependent upon the attendant or a direct-access extension.

(d) An extension fully barred from the public exchange.

A right-of-way extension is prevented from intruding on an extension engaged on a public exchange call. If required, this guard can be removed by a simple modification to the connector and callback selectors.

When specified, a maximum of three extension lines can be connected via auxiliary equipment to convert them to manual extensions. All outgoing calls are routed by the attendant whose assistance is obtained when the handset is lifted. Calls to a manual extension are completed as for a normal extension.

The exchange line circuits are arranged for working to either automatic or C.B. public exchanges.  Direct access to the public exchange from an extension is gained by dialling a single code digit. If necessary, the exchange line circuits may be arranged in two groups with different access code digits for each group.

Incoming calls are normally answered and extended by the attendant. Rapid consecutive keying of extension numbers, together with ‘camp on busy’ and intrusion facilities ensures that operating time is kept to the minimum. Positive lamp signals enable the attendant to determine when the required extension is engaged on an exchange line, locked out, or when the number keyed is spare. Supervision is given until the connection of a call to an extension is established. Each exchange line circuit has an individual appearance to allow the state of un-established calls to be easily observed. No clearing signal is provided as exchange calls are released automatically.

Outgoing exchange calls can be originated by the attendant and reverted to any extension not fully barred from the public exchange.

An extension engaged on an exchange call may make an enquiry call to another extension, without being overheard by the public exchange subscriber. The repetitive transfer of an exchange call can be accomplished without the attendant’s assistance. Attendant recall is also possible.

A guard against follow-on calls by barred direct access extensions prevents dialling into an automatic public exchange after the exchange subscriber has replaced his handset. Through clearing permits the exchange line circuit to remain in the busy condition until the public exchange equipment has released.

Direct access from all extensions to an external system, other than the public exchange, can be obtained by:-
(a) Utilizing a second group of exchange line circuits, incoming calls being answered and extended by the attendant as for public exchange calls.

(b) Replacing the second group of exchange line relay sets with tie line relay sets. Incoming calls are routed via an extension line circuit to allow direct ‘in-dialling’ to extensions. The attendant may originate a tie-line call and revert it to an extension in a similar manner to that adopted for exchange lines.

It should be noted that in either case the number of exchange line and tie line circuits that can be fitted is limited to ten. In case (b) the number of tie line circuits that can be fitted is limited to five (i.e. five exchange lines and five tie lines or six exchange lines and four tie lines, etc.).

The assistance circuit provides a both way link between extensions and the attendant. Extensions gain access to the circuit by dialling a single code digit, and the attendant may call an extension by means of the digit keys in a similar manner to that adopted for extending an exchange call.

When a fully barred extension is connected to the assistance circuit, a lamp indication is given to the attendant as a warning that the extension cannot be connected to the public exchange should such a request be made. If the circuit is engaged, a further lamp signal indicates when another assistance call is awaiting attention.

If tie lines with direct in-dialling are fitted, assistance calls can be made in the same way as if the tie line were a fully barred extension. Should the attendant need to originate a tie line call, this can be achieved directly via any free tie line circuit.

Calls from the public exchange can be dealt with by either or both of the following schemes:-

(a) Incoming calls are indicated by the ringing of bells at selected points, and any extension not fully barred from exchange lines may answer by dialling a single code digit.

(b) The instrument bells of not more than two selected extensions ring when an incoming call occurs. When the call is answered, intrusion tone is superimposed on dial tone to indicate that the night answering code should be dialled to complete the connection to the exchange line.

Should a night service extension be engaged when an incoming call occurs, intrusion tone is connected to indicate that an exchange call is waiting to be answered.


Extension lines are numbered 20-69 with digits 7, 8, 9 and 0 available for allocation as access codes to:-

(a) Exchange lines.
(b) Tie lines or a second group of exchange lines.
(c) Assistance calls.
(d) Night service answering.

Fig 2. Trunking Diagram

The trunking details of the system are as illustrated in Fig. 2.

Extension to Extension Calls:-
When an extension lifts the handset, a start signal is routed via the pulse circuit to cause the linefinder (LF), associated with a free connector, to self-drive to the relevant outlet.
This arrangement enables the traffic to be distributed evenly over the connectors and also facilitates the passing forward of a start signal to an alternative circuit should any linefinder fail to locate a calling extension within approximately 5 seconds.

Dial tone is fed from the connector. Should the caller fail to dial or cause the inter-train pause to be excessive, the connector releases after approximately 20 seconds, leaving the extension line circuit locked out until the handset is replaced.

The dialling of two digits steps the connector to the outlet associated with the required extension. The line circuit is tested and, if engaged, busy tone is transmitted to the caller; if free, ring tone is returned and ringing current is applied to the called extension’s line. If the called number is spare or locked out, the caller receives N.U. tone.

At the end of a conversation, if the caller replaces the handset first, the connector releases immediately and leaves the called extension line circuit locked out until the handset is replaced. Should the caller fail to replace the handset in approximately 20 seconds after the called extension has done so, the connector releases, thus causing the calling extension’s line circuit to be locked out until the handset is replaced.

A right-of-way extension may intrude on an engaged extension who is not connected to an exchange line by dialling the digit ‘1‘ on the receipt of busy tone. This causes a speech path to be connected to the engaged parties and intrusion tone to be transmitted to indicate the presence of the third party.

If a private conversation is desired, the engaged extensions may be requested to replace their handsets. The relevant extension is then rung automatically and the call proceeds normally.

Extension-originated Exchange Call:-
Having removed the handset and received dial tone, the extension dials the relevant code digit to gain access to an exchange line circuit. The connector steps vertically and then automatically takes one rotary step into the level concerned. Provided no other type of call is being switched at this instant, the marker circuit permits the connector to mark the outlet on which the linefinder is standing. The marker circuit initiates a start signal via the pulse circuit to cause the exchange finder (EF), associated with a free exchange line circuit, to self-drive to the outlet connected to the calling extension.

At this stage, the marker circuit releases unless another call of this type is waiting to be dealt with, in which case the switching of this call takes place before the marker is released. The average time the marker circuit is engaged on switching any one call is 500 milli-seconds.

Meanwhile, the connector releases and the extension is connected to the exchange line circuit. If the public exchange is automatic, dial tone will be returned and the required subscriber’s number may then be dialled.

Should a barred extension attempt to originate an exchange line call, N.U. tone is returned without seizure of the marker circuit. Similarly, busy tone is returned if an extension attempts a call when no free exchange line circuit is available.

Incoming Exchange Call:-
When an incoming call occurs, a ‘line’ lamp associated with the circuit’s position on the attendant’s switchboard flashes to flicker earth, and a buzzer sounds. To answer, the attendant operates the appropriate ‘ speak’ key; the lamp continues to flash but the buzzer is disconnected. Having ascertained the extension required, the attendant keys the relevant number and
this information is stored in the marker circuit. If no other type of call is being switched at this moment, the finder multiple is marked and the appropriate switch self-drives to the outlet concerned. Once this has been accomplished the marker circuit releases.

The required extension’s line circuit is tested, and if this proves to be busy, the line lamp flashes to interrupted earth. Should the extension be already engaged on an exchange line, a subsidiary lamp glows steadily. In either case, the attendant is at liberty to intrude and offer the call without being overheard by the incoming caller. Intrusion tone is heard by the engaged parties whilst the attendant is connected.

If the extension agrees to accept the call, the attendant may inform the caller accordingly and then withdraw from the circuit leaving the call camped on busy. The line lamp continues to flash to interrupted earth until the engaged parties clear, at which stage the line lamp glows steadily, the called extension is rung automatically, and ring tone is returned to the caller, all of which would have occurred earlier had the required extension line tested free initially.

When the called extension answers, the line lamp is extinguished, thus indicating that the call has been completed. At the end of the conversation, clearing is achieved automatically, thus necessitating no further action by the attendant.

If during the course of an exchange call both the attendant and the extension are connected to the circuit (e.g. if the attendant introduces a call before withdrawing) intrusion tone is transmitted to indicate the presence of the attendant.

In the event of the attendant keying a spare number or the number of a locked-out extension, a ‘number unobtainable’ lamp, in addition to the line lamp, flashes to interrupted earth. This condition may be cleared by keying another number or momentarily operating the cancel key.

Should the attendant key a code digit, a spare first digit or the number of an extension line circuit allocated to a tie line, the number unobtainable lamp glows steadily and the line lamp flashes to flicker earth. Clearance of this condition can be effected by operation of the cancel key.

originated Exchange Call:-
Firstly, the attendant momentarily operates the exchange test key, which causes the line lamp of each busy circuit not already indicating a supervisory condition to glow. A free circuit may thus be chosen and the appropriate speak key operated.

The line lamp then flashes to flicker earth and if the public exchange is automatic, dial tone is heard and the required subscriber’s number can be dialled. Thereafter, the call may be extended in the manner already described.

If the attendant wishes to release an exchange call without connecting it to an extension, this is accomplished by momentarily operating the release key prior to restoring the speak key.

Consultation Call:-
If an extension during the course of an exchange call requires to consult another extension, a momentary operation of the instrument’s transfer button causes the extension to be diverted to the call-back selector and a holding loop to be connected to the exchange line.

Dial tone is returned and the call proceeds as for an extension-to-extension call. Right-of-way is also available to extensions allowed this facility. Should any number other than that of an extension be dialled, N.U. tone is returned.

At the end of a consultation call, the originating extension again momentarily operates the transfer button to release the call-back selector and restore the connection to the exchange line.

‘Fail to dial‘ and C.S.H. ‘time-out’ guards apply to the call-back selector, as for the connector, to prevent the selector being held indefinitely in these circumstances.

Transfer Call:-
In the event of an extension requiring to transfer an exchange call, the extension to which the call is to be transferred is obtained as for a consultation call. Having achieved this, the original extension replaces the handset. This causes the marker circuit to be seized and the exchange finder to be self-driven to the outlet concerned. The exchange subscriber is connected to the new extension and the call-back selector and marker circuit release.

Special care has been taken in the circuit design to prevent an exchange call being lost owing to a fault or to mis-operation on the part of an extension during a consultation or transfer call. Should anything amiss occur, the exchange line circuit reverts to the incoming call condition and thus attracts the attention of the attendant or a night service extension.

Attendant Recall:-
If an extension engaged on an exchange call wishes to recall the attendant, this can be achieved by a double operation of the instrument’s transfer button. The call-back selector is momentarily seized and ring tone is transmitted to both the extension and the public exchange subscriber; the line lamp flashes to flicker earth and the buzzer sounds to attract the attendant.

When the attendant answers, ring tone is replaced by intrusion tone, the line lamp is extinguished and the buzzer disconnected. If only an enquiry is made, restoration of the speak key leaves the call in its original state. Should the extension request that the call be transferred, the attendant can do so.

If the extension replaces the handset before the attendant has taken any action, intrusion tone is disconnected and the line lamp flashes to flicker earth.

In the event of an extension initiating a consultation or transfer call when the call-back selector is engaged, the call is directed to the switchboard for an attendant recall as previously described.

During night service the attendant-recall facility is, of course, ineffective.

Assistance Call:-
As already explained, the assistance circuit serves as a link between extensions and the attendant. Within the limits of the facilities offered, it functions in a manner so similar to that of an exchange line circuit that further description is unnecessary except to explain the ‘call-waiting’ feature.

If an extension dials the assistance code digit at a time when the circuit is engaged, the connector steps vertically and then automatically takes one step into the level concerned. The marker allows the connector to test free; ring tone is returned to the caller and the call-waiting lamp glows on the attendant’s switchboard. During this period the marker is not prevented from dealing with other demands, but if another extension dials the assistance code digit, busy tone is returned.

When the attendant restores the’ assistance-speak’ key, the marker circuit controls the switching of the assistance circuit to the waiting caller. The connector and marker circuit then release and ring tone continues to be returned until the attendant answers.

During night service, N.U. tone is returned to any extension attempting an assistance call.

Exchange and Assistance Finder Safeguard:-
The precautions taken to prevent a linefinder driving indefinitely under fault conditions have been described earlier. In the event of an assistance or exchange finder failing to find a marked outlet, the switch concerned is arrested after approximately S seconds and the following operations occur as appropriate:-

(a) If an extension is attempting to initiate an exchange call, the marker is released and the extension receives busy tone from the connector. The exchange-finder start arrangement is such that it is unlikely that the same finder will be seized on a second attempt.

(b) In the case of an exchange transfer, the extension to which the call was being transferred is locked out; the call-back selector and marker are released, and the attendant or a night service extension is called into the circuit concerned.

(c) If an extension is attempting an assistance call, the ring tone heard by the extension is disconnected and the marker released. The connector is eventually released under C.S.H. conditions unless the extension replaces the handset earlier.

(d) Should an exchange or assistance finder fail after an extension number has been keyed by the attendant, the marker is released and the number unobtainable lamp on the attendant’s switchboard glows continuously to indicate the failure. This condition may be cleared by a momentary operation of the cancel key.

Fig 3. Exchange Line relay sets in P.A.B.X. Unit (door removed) Fig 4. Uniselectors and two-motion selectors with covers removed


Two transistor oscillators are provided, one being used for the 400 c/s tone requirements and the other for the generation of 25 c/s continuous-ringing current. Interruptions are controlled by means of interacting relays and a uniselector.

Dial Tone is provided by feeding continuous-ringing current via an attenuator network.

N.U. Tone is a continuous tone fed from the 400 c/s supply.

Busy Tone is fed from the 400 c/s supply via interrupter contacts which give tone periods of 800 milli-seconds ON and 800 milli-seconds OFF.

Intrusion Tone is also derived from the 400 c/s supply, but interrupter contacts control the tone periods to 60 milli-seconds ON, 340 milli-seconds OFF, 60 milli-seconds ON and 1140 milli-seconds OFF.

Ring Tone is provided by modulating the 400 c/s supply with the continuous-ring supply. The resultant tone is then interrupted 400 milliseconds ON, 400 milli-seconds OFF, 400 milliseconds ON and 2 seconds OFF.

Interrupted Ringing is provided for internal calls at the same intervals as Ring Tone.

Exchange Ringing is provided at the same intervals as Busy Tone and is applied in establishing exchange calls. In addition to indicating to an extension the nature of the call, it is also considered to be a more urgent calling signal.

The equipment is designed for an operating voltage of 50V but variation in voltage between 45 and 55V is permissible without operating reliability being affected.

Extension line loop resistance up to 800 ohms may be allowed with safety. The earth connection for an instrument transfer button may be obtained locally or by means of a third wire to the P.A.B.X. equipment.

Exchange and tie line loop resistance is independent of extension line limits.

Provision is made for alarms to be indicated on the attendant’s switchboard. Lamp signals are used to discriminate between urgent and non-urgent alarms, and an audible signal is used in conjunction with the urgent alarm.

Fuse, release, and ring failures are classified as urgent alarms. The non-urgent alarm covers P.G. extension line faults and operates on a delay basis. A mains-fail alarm can also be provided if required. The equipment does, however, allow selected extensions to be directly connected to the public exchange in the event of a power failure.

Every effort has been made to contain the equipment in as small a cubic capacity as possible consistent with giving easy access for maintenance purposes. As may be seen from Figs. 3 and 4, a double-sided cabinet of pressed-steel construction is used, the overall dimensions being 5’ 9” (175.3 cm) high, 2’ 11” (88.9 cm) wide and 1’ 8" (50.8 cm) deep. These measurements are inclusive and allow for the cross-connecting field cover being in position.

Fig. 3 shows the mounting arrangement of the extension line circuits, assistance circuit, marker circuit, and eight exchange line circuits with positions for a further two exchange line circuits. Fuse panels, heat coils and a routine test jack are also mounted on this face.

On the opposite side, as may be seen from Fig. 4, are the uniselectors employed as assistance, line, and exchange, finders. Below these are the extension line and miscellaneous connection strips, connectors, and call-back selector. Also included at the lower right-hand corner are the relay sets accommodating the ringing and tone generating equipment and the manual extension auxiliary circuits. Selector and relay-set covers are provided, but are shown removed in Fig. 4 in order to give some idea of the amount of equipment used by the various circuits.

Perspex doors provide protection against dust and allow the functioning of the equipment to be observed. The cross-connecting field enables access to be obtained to extension and exchange line terminations for testing and re-routing purposes without disturbing the rest of the equipment.

The outer parts of the unit are finished in opaline green and the inner details in a light straw colour.

Fig 5. The Attendants Switchboard Fig 6. Method of replacing a faulty lamp

Fig. 5 illustrates the attendant’s switchboard. The shell is the same pattern as that adopted for our cordless switchboard described in detail in Bulletin No. 35. It occupies no more desk space than an office typewriter and is 13” (33 cm) Wide, 13” (33 cm) deep and 8.5” (21.6 cm) high.

The face panel has been carefully designed to keep operating procedure as simple as possible. At the top of the board may be seen the special supervisory and alarm lamps described earlier. The first seven keys on the upper row are double throw, each direction being associated with a lamp peculiar to an exchange line circuit, the assistance circuit or a manual extension. A further key is used for the control of manual calls, the other two keys being associated with alarms and night service working. At the bottom of the panel are the digit keys used for the completion of internal calls. The opposite throw of several of the digit keys is utilized for ‘ splitting’, intrusion and other requirements. The dial is conveniently placed for the attendant to originate external calls.

The loosening of a captive screw on either side of a lamp strip (Fig. 6) enables the lamp assembly to slide forward, thus permitting a lamp to be removed without the need for a lamp extractor. In the event of the exchange line circuits being split into two groups, coloured lamp caps are fitted to distinguish the circuits associated with each group.

The switchboard is finished in two-tone grey which readily blends with most modern office furnishings.

Apparatus of proven reliability is used. It includes British Post Office standard type heavy-duty uniselectors, Type 4000 (or alternatively, Type 2000) two-motion selectors, and ‘major’ and ‘minor’ relays, as well as ‘twin type line relays, together with the high-speed and thermal types required for special circuits.

The equipment has full tropical finish, whilst p.v.c. insulated wire is used for the connections; thus, with the additional protection afforded by the apparatus covers, and being totally enclosed in a dust-proof cabinet, the whole is adequately safeguarded.

P.A.B.X. 10 + 50 has capacity for 10 exchange line, 50 extension line and 7 connector circuits. The stock equipment is for 4 exchange lines, 30 extension lines and 4 connectors, together with the call-back selector, assistance circuit and other common items. Extension by the addition of line circuit and jack-in equipment is a simple matter as the unit is fully wired.

A mains-operated rectifier unit designed for wall mounting adjacent to the equipment unit meets the needs of most installations. If preferred, batteries and a suitable charger are supplied.

The provision of facilities of doubtful efficacy has been avoided. Well-established switching principles are employed and the exterior appearance has received as much thought in design as the most intricate circuit detail.

It is confidently anticipated that organizations which avail themselves of this new P.A.B.X. will be satisfactorily served for many years.

See also N22845

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Last revised: November 12, 2020